There is currently great interest in enabling several radio transmitters, which are not collocated, to cooperate in communicating a message to a receiver or several receivers. This concept is known as cooperative communications and it offers several advantages including increased communication range, increased robustness to channel fading and other impairments (e.g., jamming) and robustness in multi-hop routes in networks with little or no centralized control (i.e., so called ad-hoc networks).
Cooperative communication is conceptually similar to transmission from multiple antennas that are collocated, or collocated antennas arrays (commonly referred to as antenna arrays). In this context the term collocated means that the transmitters, which may be complete radios or antennas, amplifiers and other portions of a radio, are located in close enough proximity of each other in order to enable simple, joint control of these components. Although in most abstract models the difference between collocated and non-collocated transmitters is minimal, this distinction is critically important in practice since methods developed for collocated arrays assume various levels of coordination between the transmitted signals. This distinction is illustrated in FIG. 1 through FIG. 3. In FIG. 1, a collocated array is shown, where the individual transmitters are connected by a reliable communications bus that can be used to send low-latency, highly reliable coordination communication signals. The coordination required for the collocated array is shown by a connecting line which may physically correspond to a highly-reliable and synchronized communications bus (e.g., a bus within a chassis housing several boards, wired connections on a single board, etc.).
FIG. 2 shows a coordinated non-collocated array with a method for coordination communication. This method for coordination communication is expected to be less capable than the bus shown in FIG. 1. For example, coordination communication messages in FIG. 2 may experience delay, errors, and limited data rates. FIG. 3 illustrates an uncoordinated cooperative communication system wherein each transmitter operates completely autonomously from the others and no coordination communication channel is assumed.
Coordination between multiple transmitters that are not collocated is difficult to achieve in practice. Even in approaches where the required level of coordination between transmitters is possible, the application scenario may make it prohibitive. Some specific examples of coordination include                Knowledge of the number of transmitters with this information available at one or more transmitters and/or the receiver;        An ordering of the transmitters known to the transmitters;        Knowledge of the channel characteristics from a given transmitter to the receiver by one or more of the transmitters;        Synchronization of the transmitters carrier frequency, phase, gain, and/or timing.        
Different methods proposed for cooperative communications require some or all of these types of coordination as will be illustrated in the examples that follow.
When multiple transmitters send the same signal to a common receiver, they are superimposed at the receiver into a composite signal. This can result in constructive interference, in which the power of the composite signal is larger than that of any of the individual signals or it may result in destructive interference, in which the power of the composite signal has power less than one of the individual received signals. This is because radio transmissions are typically performed by modulating a sinusoidal carrier signal. When several such signals arrive at different relative phases, they create an interference pattern. If the phases of the signals are equal (or nearly equal), the signals will constructively interfere. This is commonly referred to as coherently combining the signals. The worst case scenario is that the signals arrive with opposite phases. For example, if two sinusoidal signals with the same frequency and amplitude, but with a 180 degree phase difference are combined, the composite signal is zero, i.e., this is complete destructive interference. These concepts are illustrated in FIG. 4 where the sinusoidal signals are represented by their amplitude and phase in a plane (i.e., the inphase/quadrature plane).
There are several aspects of the system that may affect the characteristics of the received signal such as amplitude, frequency, phase, etc. These include the reference phase and frequency of the transmitter and the propagation channel. The propagation channel (or, for brevity, the channel) naturally adds a phase offset due to the time it takes for the waveform to propagate from the transmitter to the receiver. The channel, in open space, typically attenuates the signal amplitude. In some cases, relative mobility between the transmitter and the receiver will cause a shift in frequency (Doppler shift). In some other cases, multiple paths for propagation (multipath) will introduce Rayleigh or Ricean fading effects. As consequence, the channel between each transmitter and the receiver will face several impairments such as signal attenuation along the transmission path, phase offset due to propagation delay, carrier frequency offset due to Doppler shift, frequency selective fading due to multipath effects.
Consider, for example, two transmitters sending the same signal without coordination. In such cases, the two signals will be superimposed with random carrier phases. This results in a probability distribution on the received signal pattern. There is some probability of constructive interference and some probability of destructive interference. If the channel and transmitter characteristics are time invariant (e.g., stable oscillators, no mobility, etc.) then the realized interference pattern will be stable. This means that the effective received signal-to-noise ratio (SNR) will be fixed and under a large number of operational scenarios, this SNR will be too small for effective communication. This will be referred to as the naïve uncoordinated method. Note that even in this case, it may be assumed that the signals are synchronized in time. This assumption may be relaxed to some extent depending on the channel model considered as will be explained later.
This suggests that coordination between transmitters is desirable for effective cooperative communication. In the ideal case, the channel characteristics from each transmitter to the receiver would be available at each transmitter. For example, transmitter 1 in FIG. 2 would have knowledge of channel 1. Each transmitter would then compensate for the channel effects (e.g., a carrier phase rotation) so each transmitted signal would arrive at the receiver aligned in phase and frequency (e.g., all arrive with zero-phase difference). In this case, perfectly constructive interference would be obtained. This method will be referred to as distributed beam forming since it is analogous to coherent beam-forming methods in collocated arrays. In practice, not only do the transmitters need to know the channel characteristics, but the transmitter oscillators must be very stable and controlled to maintain this coherent relationship. This is often challenging even in collocated arrays, and therefore extremely challenging in the case of non-collocated transmitters. This issue is further exacerbated by the presence of mobility between transmitters and the receiver, mobility in the propagation environment, and/or the desire to have inexpensive RF circuit components.
In collocated arrays, the method of space-time coding provides an alternative to beam-forming. Specifically, space-time coding typically does not require the transmitters to know the channel characteristics from transmitter to the receiver. An example of space-time coding is the simple, effective Alamouti code. In this case, two transmitting antennas are used to send two data symbols consecutively in a coordinated manner. Specifically, transmitting antenna 1 sends the complex-baseband symbol s[1], followed by s[2]. During the same time, synchronized at the symbol time level, transmitting antenna 2 sends symbols −s[2], followed by s[1]. Here s* denotes the complex conjugate of the complex baseband symbol s. This Alamouti space-time code is an example of a code with two antennas and a block length of two symbols. By transmitting data in this manner, diversity is obtained, i.e., it ensures that destructive interference over the entire block will not occur. Note that the channels are used twice to communicate 2 symbols s[1] and s[2]. Note that transmitted signals are described in a complex baseband signal notation. The actual transmitted signal is related to the complex baseband signal by the relation: z(RF)=Re{z[t]exp(j2πfct)} where Re(.) represents the real part of a complex variable, and fc denotes the carrier frequency. Baseband signals may generally be characterized by digital data that may have been encoded, interleaved, and/or symbol mapped, and may include frequencies that are equal to or very near zero. Passband signals or any modulated baseband signals are signals that are in the radio frequency (RF) ranges (3 kHz to 300 GHz) and can be transmitted wirelessly. Passband signals may be intermediate RF signals that will be modulated over a higher frequency for transmission or they may be RF signals that can be directly transmitted. Passband signals are often modeled mathematically as an equivalent complex-value baseband signal. In practice, a complex baseband signal can be presented in the form of z(t)=I[t]+jQ[t] where I[t] is the inphase signal and Q[t] is the quadrature signal. The physical passband signal corresponds to I[t] cos(2πfct)−Q[t] sin(2πfct) or z(RF)=Re{z[t]exp(2πfct)}. In the following description, passband signals, i.e., both transmitted and received modulated signals, data, or symbols, will be modeled as an equivalent complex-value baseband signal for the purpose of illustration.
Such space-time coding methods may also be considered for the case of non-collocated arrays. This method is referred to as distributed space-time coding. Distributed space-time coding thus requires several levels of coordination. First, the transmitters and the receiver must have knowledge of the number of transmitters. Second, an ordering of the transmitters must be established. This ordering is required so that the appropriate symbol sequence can be assigned to each transmitter. In the space-time coding literature, it is conventional to express the space-time code as a matrix of symbols, each row corresponding to the sequence of symbols to be transmitted. Thus, the ordering of transmitters corresponds to assigning a specific row of the space-time code matrix to each transmitter. A third level of coordination is time synchronization. The transmitters must be synchronized at the symbol time level and also must be synchronized at the space-time code block level. This level of coordination is likely easier to achieve in practice than that required for distributed beam-forming. A small loss in performance will be suffered relative to distributed beam-forming, however, since perfect constructive interference is not achieved.
The level of coordination required for distributed beam-forming and distributed space-time coding is undesirable for a number of practical systems of interest. In fact, it may be impossible to provide this level of coordination in many cases. One specific example is the case of an ad-hoc, mobile network of radios. Consider the case where one node (a node means the same as a user, a transceiver device, a mobile handset, a transmitter, a receiver, or a base station hereinafter) in such a network transmits a message (in the form of a complex baseband signal) that is received by two or more other nodes; these are hop-1 nodes. It is then desirable for these hop-1 nodes that received the initial message to cooperate to send the message to another set of nodes, i.e., hop-2 nodes. This is illustrated in FIG. 5. Consider the case when the nodes are moving and when it is necessary to disseminate the message through the network with low-delay. These requirements are consistent with applications such as emergency response and tactical squadron communications.
In this case, it is impractical or impossible for the nodes to be coordinated effectively in a rapid manner. For example, in order to coordinate for distributed space-time coding, the hop-1 nodes that successfully received the initial transmission would need to send an acknowledgement message back to the originating transmit node. This transmit node would then send a second message to indicate the number of nodes that received the initial transmission and some ordering, possibly based on identification information included in the acknowledgement messages. Since the nodes are mobile and the wireless propagation channel is inherently random, it is possible that during this round of coordination transmission errors can occur. For example, suppose 4 nodes successfully received the original transmission and sent acknowledgements, but of those 4, only 3 successfully receive the order assignment. This will result in transmission of 3 of the 4 required rows of a space-time code matrix possibly resulting in a significant loss in performance. Even if the coordination communication is perfect, it requires additional delay which is undesirable in many applications.
Using space-time beam-forming requires similar coordination communication to establish knowledge of the channel conditions at the nodes to be coordinated. Specifically, each hop-1 node would need to identify the channel from itself to a hop-2 node. Conceptually, this could be achieved with a round of coordination communication. Then, the hop-1 nodes could use distributed beam-forming to communication with a hop-2 node. This is difficult in practice because the effective channel characteristics are likely to change during this process due to factors such as node mobility and oscillator drift. Another limitation of this approach is that it may be desired to relay the message from hop-1 nodes to many hop-2 nodes, i.e., not just one hop-2 node. This is the case when, for example, it is desired to have several hop-2 nodes cooperate to send the message further out into the network. With distributed beam-forming, however, it is difficult for cooperating transmitters to simultaneously align their signals coherently at more than one node. Thus, in the scenario considered, a separate round of coordination communication and coordinated cooperative transmissions would be required for each hop-2 node. This introduces further delay and complexity.
In the given exemplary scenario, the less coordination required the better. For example, of the techniques described, one may select the naive uncoordinated method. In that case there is no coordination communication required and cooperation occurs autonomously and without additional delay. Specifically, the hop-1 nodes would simply transmit the message and the resulting composite signal would be received at each hop-2 node. The drawback with this approach is that there is a significant probability of a stable destructive combining pattern at specific nodes that will prevent successful reception.
Thus, the number of hop-2 nodes will be reduced relative to that in the case of distributed space-time coding or distributed beam-forming.
This establishes the need for a method of performing cooperative communications with no coordination, but in a manner that is robust against stable destructive interference patterns at the receiver.